Author Topic: [SOLVED] Prestine Fluke 75: is it worth searching for a new U1 chip?  (Read 1746 times)

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Offline wwjd1924Topic starter

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I bought a Fluke 75 in very, very good condition.  only 1 tiny little scratch on the display plastic.
But I knew it was not working and I hoped to revive it but I failed.

I cannot get a value displayed at VDC or VAC setting.

- I did resolder the U1 contacts
- cleaned S1
- R1 is not open: ~1K
- RV1 and RV2 not shorted

- checked over and over again the pcb with a microscope
- followed the troubleshooting procedure:
-- The VDC signal path check procedure 3-21 of the service manual failed: Instead of 200mV I measured 0.2mV

Also when I apply a voltage on the input of the MM, the current limiting kicks in.
I did not set the current on the PS higher than 100mA but is should not short when measuring.

In the Ohms setting I get 2.7xx MOhms when shorted


I still like the Fluke! I could not afford it 40 years ago. I would love to get it working again!

Any suggestions??

Thanks!
« Last Edit: February 18, 2022, 06:24:38 pm by wwjd1924 »
 

Offline abdulbadii

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Re: Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #1 on: January 23, 2022, 01:14:11 am »
Dutch not importing cheap Taiwan/China/Korea products?
we'll get 2-3 times better quality for the same cost
 
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Offline rsjsouza

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Re: Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #2 on: January 23, 2022, 01:26:51 am »
If you are suspecting the main ICs are busted, perhaps consider buying another meter as a parts donor?

Dutch not importing cheap Taiwan/China/Korea products?
we'll get 2-3 times better quality for the same cost
Unless the OP paid hundreds of dollars for it, there's no comparison in quality with China-made DMMs. Taiwan-based Brymen is fine, as well as Malaysia-made Keysight. UEi is the only one I know from Korea, and they are not good.
Vbe - vídeo blog eletrônico http://videos.vbeletronico.com

Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 
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Offline tooki

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Re: Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #3 on: January 23, 2022, 02:20:09 am »
Dutch not importing cheap Taiwan/China/Korea products?
we'll get 2-3 times better quality for the same cost
No. You get 2-3 times as many features, but with lower quality, at the same cost. Do not confuse features with quality.
 
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Offline lowimpedance

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Re: Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #4 on: January 23, 2022, 02:43:25 am »
As the  'OP' has made measurements  on this  meter  the  assumption  can be made  that  he has other  meters,  so this post is more  about  resurrection  of a classic  FLUKE rather  than  having a need for  any  specific function.

@OP, when  you say section  3.21 failed do you  mean you  measured  0.2 mV at Z1 pin 3 or further  along  at the filter ?.
One other quick  check to make sure the  crystal  y1 is oscillating.
However  if your  sure U1 is dead then  as suggested  your  only  bet would  be a junker unit , keep an eye out on your  favourite  'auction  site' or perhaps  put a WTB request  in the BUY/SELL/WANTED board and hopefully  one will  turn  up.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 
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Offline wwjd1924Topic starter

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Re: Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #5 on: January 23, 2022, 02:01:21 pm »
Thanks to all who replied!

Yes I have another DVM: a Fluke 179...  and yes I know this is an overkill for a hobbyist...  :palm:  :-DMM
But I love well made products...

This Fluke 75 meter, as I said, was a unreachable dream decades ago...
Even if I cannot revive it, I still am glad to own it now... Silly maybe?!

I can understand that putting time into this for members of this great forum is a waste of time but I do appreciate any help!

To answer the question of @lowimpedance:

@OP, when  you say section  3.21 failed do you  mean you  measured  0.2 mV at Z1 pin 3 or further  along  at the filter ?.
One other quick  check to make sure the  crystal  y1 is oscillating.
However  if your  sure U1 is dead then  as suggested  your  only  bet would  be a junker unit , keep an eye out on your  favourite  'auction  site' or perhaps  put a WTB request  in the BUY/SELL/WANTED board and hopefully  one will  turn  up.

- I will certainly look for another unit for spare parts... I saw a Fluke 23 in working order for 57 euro's...  But a broken unit will be cheaper hopefully

- the DVM has problems coming out of the display selftest now, after 3 minutes it passes that 'test'...
- The VAC and VDC modes give a overrange 'OL' continually
- The oscillator is working!  I measured it: 32.76Khz  but the voltage pp was 2.2V instead of the 600mV stated?
- pin 1 of the input divider Z1 is 2V DC as it should be so no shorted MOV or open R1
- pin 3 of Z1 should be 200Mv but is basically zero!  Yesterday I said I measured 0.2mV  but in the Fluke 179 mV range that is just noise

I will investigate the divider, solder it out and check it.
after that I think removing the S1 rotary switch and checking it will be useful.

again, thanks to everyone willing to think with me to restore this beautiful DVM.




 

Offline wwjd1924Topic starter

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Re: Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #6 on: January 23, 2022, 08:03:19 pm »
Update:

Today I desoldered all capacitors (except the ones around the U2 chip), the S1 switch and the voltage divider.  Checked all of them: The S1 switch was OK as was the Z1 voltage divider.
The capacitors are as good as new.
Measured all resistors and checked the diode and zener: all ok...

Measured all pcb traces going from the U1 chip, one by one: no problems there also.

So I must conclude that the U1 chip is in need of replacement.

Could the cause of the blow U1 chip be that the rotary switch S1 can be rotated 360 degrees?  I don't know if there should be end stops?



 

Offline Shock

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Re: Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #7 on: January 24, 2022, 05:13:12 am »
They are nice meters, I have a few of the original series. The Fluke 27/FM have more features though.

The cover/function knob prevents the switch from rotating fully. Not sure if it damages anything but it's not a good idea to rotate it past that point if broken. Here is a repair Mr. ModemHead did by using a small screw with the top filed. I'd probably use part of a small motor shaft or pinch roller pin from an old cassette deck.

Soldering/Rework: Pace ADS200, Pace MBT350
Multimeters: Fluke 189, 87V, 117, 112   >>> WANTED STUFF <<<
Oszilloskopen: Lecroy 9314, Phillips PM3065, Tektronix 2215a, 314
 
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Offline wwjd1924Topic starter

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Re: Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #8 on: January 24, 2022, 01:11:46 pm »
Thank for the tip, I will certainly fix that knob...!
 

Offline wwjd1924Topic starter

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Re: [SOLVED] Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #9 on: February 18, 2022, 06:33:32 pm »
A-MA-ZING recovery...

The meter is as new again...  Fully unexpected it started working again...

I removed the U1 A/D converter chip and replaced it with another U1 chip from a donor Fluke 75 but that didn't work at all.

Then, to at least have some reading I resoldered the original U1 and to my amazement the meter started to work again!

At first the reading were a bit off and when measuring a voltage, the reading was not stable but lowering slowly which is strange of course, as you were measuring a discharging capacitor.

Then I reheated/resoldered all pins of the U1 chip again and after that the readings were stable and correct.
After calibrating it with R8, the Fluke 75 is now *very* accurate and as new...

I suspect the meter never has worked because the case and acrylic display cover are without any scratches

Maybe the heat from removing the U1 chip with a hot air rework station and resoldering has revived it?
 :-DMM

 
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Offline BILLPOD

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Re: [SOLVED] Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #10 on: February 18, 2022, 07:33:56 pm »
Good Morning Wwjd1924,  Congratulations on your successful repair, (great feeling, isn't it?).  And thank you for coming back to post what you did, as it surely will help others with similar problems. :-+
 
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Offline retiredcaps

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Re: [SOLVED] Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #11 on: February 18, 2022, 08:26:13 pm »
Maybe the heat from removing the U1 chip with a hot air rework station and resoldering has revived it?
Member Excavatoree has mentioned that sometimes resoldering U1 makes the unit work again.
 
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Offline wwjd1924Topic starter

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Re: [SOLVED] Prestine Fluke 75: is it worth searching for a new U1 chip?
« Reply #12 on: February 19, 2022, 04:59:39 pm »
Good Morning Wwjd1924,  Congratulations on your successful repair, (great feeling, isn't it?).  And thank you for coming back to post what you did, as it surely will help others with similar problems. :-+

That was one of the first things I did, but that didn't work in my case...
The hot air rework has heated the chip much more so just maybe that's the explanation in my case??
 


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